Conventional strawberries in Watsonville, California. Researchers say organic foods are no more nutritious and no less likely to be contaminated.Credit
Jim Wilson/The New York Times

Does an organic strawberry contain more vitamin C than a conventional one?

Maybe — or maybe not.

Stanford University scientists have weighed in on the “maybe not” side of the debate after an extensive examination of four decades of research comparing organic and conventional foods.

They concluded that fruits and vegetables labeled organic were, on average, no more nutritious than their conventional counterparts, which tend to be far less expensive. Nor were they any less likely to be contaminated by dangerous bacteria like E. coli.

The researchers also found no obvious health advantages to organic meats.

Conventional fruits and vegetables did have more pesticide residue, but the levels were almost always under the allowed safety limits, the scientists said. The Environmental Protection Agency sets the limits at levels that it says do not harm humans.

“When we began this project, we thought that there would likely be some findings that would support the superiority of organics over conventional food,” said Dr. Dena Bravata, a senior affiliate with Stanford’s Center for Health Policy and the senior author of the paper, which appears in Tuesday’s issue of the Annals of Internal Medicine. “I think we were definitely surprised.”

The conclusions will almost certainly fuel the debate over whether organic foods are a smart choice for healthier living or a marketing tool that gulls people into overpaying. The production of organic food is governed by a raft of regulations that generally prohibit the use of synthetic pesticides, hormones and additives.

The organic produce market in the United States has grown quickly, up 12 percent last year, to $12.4 billion, compared with 2010, according to the Organic Trade Association. Organic meat has a smaller share of the American market, at $538 million last year, the trade group said.

The findings seem unlikely to sway many fans of organic food. Advocates for organic farming said the Stanford researchers failed to appreciate the differences they did find between the two types of food — differences that validated the reasons people usually cite for buying organic. Organic produce, as expected, was much less likely to retain traces of pesticides.

Organic chicken and pork were less likely to be contaminated by antibiotic-resistant bacteria.

“Those are the big motivators for the organic consumer,” said Christine Bushway, the executive director of the trade association.

The study also found that organic milk contained more omega-3 fatty acids, which are considered beneficial for the heart.

The Stanford researchers said that by providing an objective review of the current science of organic foods, their goal was to allow people to make informed choices.

In the study — known as a meta-analysis, in which previous findings are aggregated but no new laboratory work is conducted — researchers combined data from 237 studies, examining a wide variety of fruits, vegetables and meats. For four years, they performed statistical analyses looking for signs of health benefits from adding organic foods to the diet.

Photo

Organic apples on display in a market in Glen Cove, New York.Credit
Kathy Kmonicek for The New York Times

The researchers did not use any outside financing for their research. “I really wanted us to have no perception of bias,” Dr. Bravata said.

One finding of the study was that organic produce, over all, contained higher levels of phosphorus than conventional produce. But because almost everyone gets adequate phosphorus from a wide variety of foods, they said, the higher levels in the organic produce are unlikely to confer any health benefit.

The organic produce also contained more compounds known as phenols, believed to help prevent cancer, than conventional produce. While the difference was statistically significant, the size of the difference varied widely from study to study, and the data was based on the testing of small numbers of samples. “I interpret that result with caution,” Dr. Bravata said.

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Other variables, like ripeness, had a greater influence on nutrient content. Thus, a lush peach grown with the use of pesticides could easily contain more vitamins than an unripe organic one.

The study’s conclusions about pesticides did seem likely to please organic food customers. Over all, the Stanford researchers concluded that 38 percent of conventional produce tested in the studies contained detectable residues, compared with 7 percent for the organic produce. (Even produce grown organically can be tainted by pesticides wafting over from a neighboring field or during processing and transport.) They also noted a couple of studies that showed that children who ate organic produce had fewer pesticide traces in their urine.

The scientists sidestepped the debate over whether the current limits are too high. “Some of my patients take solace in knowing that the pesticide levels are below safety thresholds,” Dr. Bravata said. “Others have questioned whether these standards are sufficiently rigorous.”

Dr. Bravata agreed that people bought organic food for a variety of reasons — concerns about the effects of pesticides on young children, the environmental impact of large-scale conventional farming and the potential public health threat if antibiotic-resistant bacterial genes jumped to human pathogens. “Those are perfectly valid,” she said.

The analysis also did not take factors like taste into account.

But if the choice were based mainly on the hope that organic foods would provide more nutrients, “I would say there is not robust evidence to choose one or the other,” Dr. Bravata said.

The argument that organic produce is more nutritious “has never been major driver” in why people choose to pay more, said Ms. Lunder, the Environmental Working Group analyst.

Rather, the motivation is to reduce exposure to pesticides, especially for pregnant women and their young children. Organic food advocates point to, for example, three studies published last year, by scientists at Columbia University, the University of California, Berkeley, and Mount Sinai Hospital in Manhattan. The studies identified pregnant women exposed to higher amounts of pesticides known as organophosphates and then followed their children for years. In elementary school, those children had, on average, I.Q.’s several points lower than those of their peers.

Critics of the Stanford study also argue that lumping all organic foods into one analysis misses the greater benefits of certain foods. For example, a 2010 study by scientists at Washington State University did find that organic strawberries contained more vitamin C than conventional ones.

Dr. Crystal Smith-Spangler, another member of the Stanford team, said that the strawberry study was erroneously left out but that she doubted it would have changed the conclusions when combined with 31 other studies that also measured vitamin C.

A version of this article appears in print on September 4, 2012, on Page A20 of the New York edition with the headline: Stanford Scientists Cast Doubt on Advantages of Organic Meat and Produce. Order Reprints|Today's Paper|Subscribe